1. Field of the Invention
This invention relates to a new process for the preparation of new aromatic polyisocyanates containing urea and/or biuret groups by the reaction of diisocyanates with subequivalent quantities of primary and/or secondary diamines, in which the diisocyanates and/or diamines used as reactants consist wholly or in part of special diisocyanates described in more detail below and/or special diamines described in more detail below, and to the use of the products of the process as isocyanate components in the production of polyurethanes.
2. Description of the Prior Art
Polyisocyanates containing urea and/or biuret bonds are known (see e.g., "Polyurethanes", Chemistry and Technology, Part I, by Saunders and Frisch, Interscience Publishers, (1962), pages 190 et. seq.).
Such urea and/or biuret bonds are normally obtained by the action of water on polyisocyanates, which entails the loss of valuable isocyanates.
British Pat. No. 1,078,390 describes the formation of biurets directly from diamines and aromatic polyisocyanates by carrying out this reaction in solvents having a boiling point below the boiling point of the isocyanate, e.g., chloroform. The disadvantage of this process is that the reaction must be followed by removal of the solvent by distillation. Direct reaction of the diamines given in the examples with the isocyanates described is not possible without solvents since the difficultly soluble polyureas which are formed instantly prevent further reaction of the isocyanate.
German Offenlegungsschrift No. 1,963,190 also describes a process in which aromatic polyamines containing primary amino groups can be directly reacted with polyisocyanates to form soluble biurets without the aid of solvents if the polyamines with primary amino groups have an attenuated nucleophilic character. The reactivity of the amines with isocyanates must be attenuated to such an extent that a homogeneous mixture of amine and isocyanate can be prepared before the two components begin to react. This requires the employment of very elevated temperatures, with the attendant risk of undesirable side reactions and the precipitation of solid constituents.
It was therefore an object of the present invention to provide a new process for the preparation of stable, liquid aromatic polyisocyanates containing urea and/or biuret groups from diamines and diisocyanates which may be carried out in the liquid phase without the aid of auxiliary solvents but at the usual temperatures employed for preparations without any insoluble urea and/or biuret constituents being found to precipitate.
This problem could be solved by the process according to the invention described below.